Thermal Properties of High Power Laser Bars Investigated by Spatially Resolved Thermoreflectance Spectroscopy

In this work we present results of the analysis of thermal properties of high-power laser bars obtained by spatially resolved thermoreflectance (TR) spectroscopy. Thermoreflectance is a modulation technique relying on periodic facet temperature modulation induced by pulsed current supply of the laser. The periodic temperature change of the laser induces variation of the refractive index and consequently modulates probe beam reflectivity. The technique has a spatial resolution of about ~1 $\mu$m and can be used for temperature mapping over 300 $\mu$m x 300 $\mu$m area. Information obtained in these experiments provide an insight into thermal processes occurring at devices' facets and consequently lead to increased reliability and substantially longer lifetimes of such structures.Comment: Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions

The growth of ZnO crystals from the melt

The peculiar properties of zinc oxide (ZnO) make this material interesting for very different applications like light emitting diodes, lasers, and piezoelectric transducers. Most of these applications are based on epitaxial ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the thermochemical properties of ZnO make the growth of single crystals difficult: the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals for technical applications can only be grown either by a hydrothermal method or from "cold crucibles" of solid ZnO. Both methods are known to have significant drawbacks. Our thermodynamic calculations and crystal growth experiments show, that in contrast to widely accepted assumptions, ZnO can be molten in metallic crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result is believed to offer new perspectives for ZnO crystal growth by established standard techniques like the Bridgman method.Comment: 6 pages, 6 figures, accepted for J. Crystal Growt

Growth of Oxide Compounds under Dynamic Atmosphere Composition

Commercially available gases contain residual impurities leading to a background oxygen partial pressure of typically several 10^{-6} bar, independent of temperature. This oxygen partial pressure is inappropriate for the growth of some single crystals where the desired oxidation state possesses a narrow stability field. Equilibrium thermodynamic calculations allow the determination of dynamic atmosphere compositions yielding such self adjusting and temperature dependent oxygen partial pressures, that crystals like ZnO, Ga2O3, or Fe{1-x}O can be grown from the melt.Comment: 4 pages, 3 figures, talk on CGCT-4 Sendai, May 21-24, 200

Desempenho de genótipos de canola em Goiás, em 2004.

bitstream/CNPT-2010/40483/1/p-co118.pd

Spectroscopic signatures of a bandwidth-controlled Mott transition at the surface of 1T-TaSe2_2

High-resolution angle-resolved photoemission (ARPES) data show that a metal-insulator Mott transition occurs at the surface of the quasi-two dimensional compound TaSe$_2$. The transition is driven by the narrowing of the Ta $5d$ band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up.Comment: 4 pages, 4 figures; one modified figure, added referenc

A complementary neutron and anomalous x-ray diffraction study

Distinguishing the scattering contributions of isoelectronic atomic species by means of conventional x-ray- and/or electron diffraction techniques is a difficult task. Such a problem occurs when determining the crystal structure of compounds containing different types of atoms with equal number of electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid solution. Our model is based on neutron and anomalous x-ray diffraction experiments. These complementary techniques allow the separation of scattering contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly identically in monoenergetic x-ray diffraction experiments. We have found that CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification exhibits a modified stannite structure (space group I4¯2m). Different occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies have been found at three different cationic positions of the structure (Wyckoff sites 2a, 2b, and 4d) depending on the composition of the compound. Significantly, Cu+ does not occupy the 2b site for the In-free compound, but does for the In-containing case. Structural parameters, including lattice constants, tetragonal distortions, and occupation factors are given for samples covering the entire range of the CuIn3Se5–CuGa3Se5 solid solution. At the light of the result, the denotation of Cu-poor 1:3:5 compounds as chalcopyrite-related materials is only valid in reference to their composition

Nitrogen transfer from forage legumes to nine neighbouring plants in a multi-species grassland

Legumes play a crucial role in nitrogen supply to grass-legume mixtures for ruminant fodder. To quantify N transfer from legumes to neighbouring plants in multi-species grasslands we established a grass-legume-herb mixture on a loamy-sandy site in Denmark. White clover (Trifolium repens L.), red clover (Trifolium pratense L.) and lucerne (Medicago sativa L.) were leaf-labelled with 15N enriched urea during one growing season. N transfer to grasses (Lolium perenne L. and xfestulolium), white clover, red clover, lucerne, birdsfoot trefoil (Lotus corniculatus L.), chicory (Cichorium intybus L.), plantain (Plantago lanceolata L.), salad burnet (Sanguisorba minor L.)and caraway (Carum carvi L.) was assessed. Neighbouring plants contained greater amounts of N derived from white clover (4.8 gm-2) compared with red clover (2.2 gm-2) and lucerne (1.1 gm-2). Grasses having fibrous roots received greater amounts of N from legumes than dicotyledonous plants which generally have taproots. Slurry application mainly increased N transfer from legumes to grasses. During the growing season the three legumes transferred approximately 40 kg N ha-1 to neighbouring plants. Below-ground N transfer from legumes to neighbouring plants differed among nitrogen donors and nitrogen receivers and may depend on root characteristics and regrowth strategies of plant species in the multi-species grassland

Photoluminescence lineshape of ZnO

The merger of the absorption coefficient dispersion, retrieved from transmission by the modified Urbach rule introduced by Ullrich and Bouchenaki [Jpn. J. Appl. Phys. 30, L1285, 1991], with the extended Roosbroeck-Shockley relation reveals that the optical absorption in ZnO distinctively determines the photoluminescence lineshape. Additionally, the ab initio principles employed enable the accurate determination of the carrier lifetime without further specific probing techniques